1. Field of the Invention
The invention relates to a femoral head center identifying apparatus for identifying the position of a femoral head center within a plane parallel to a frontal plane during total knee replacement surgery.
2. Description of Related Art
Generally speaking, when viewing an X-ray image (frontal plane image) of the anterior surface of a lower extremity, the weight bearing line of the human body conceivably passes through the central region of the knee joint, if the angle of intersection between the functional axis of the anterior surface of the femur which axis connects the femoral head center to the knee joint center, and the functional axis of the anterior surface of the os cruris which axis connects the knee joint center to the talus center (this angle is hereinafter referred to as “lower extremity alignment angle”) is zero degree. Therefore, such a lower extremity alignment is regarded as an ideal alignment.
For the above reason, the goal of total knee replacement that involves replacement of at least a part of a knee joint with a joint prosthesis is to make the lower extremity alignment angle after surgery closest to zero degree, and there have been heretofore developed various surgical techniques and instruments intended to obtain angles within a range of ±3 degrees which are recognized as “optimum angles”. One such example is the navigation system disclosed in JP-T-2008-515601 that is a computer-aided system for performing accurate total knee replacement. Use of this navigation system enables accurate identification of the position of a femoral head center during surgery with the aid of a computer, which makes it possible to install a block for bone resection in place in the femur while checking the position of the femoral head center so that accurate bone resection can be performed on the distal end of the femur. It is accordingly possible to place a joint prosthesis at the distal end of the femur that has been resected and, in consequence, excellent lower extremity alignment can be established with a post-surgery lower extremity alignment angle falling with the range of ±3 degrees.
This navigation system however has a problem that it incurs enormous development costs, resulting in a considerable increase in surgical expense. In addition, navigation marks (benchmarks) for use in identification of the position of the femoral head center need to be fixed to the femur, using pins which may cause damage to the femur.
To solve the above problems, there has been recently developed an inexpensive surgical technique called “shape matching” that uses no navigation systems. In this surgical technique, image data of the hip joint region and knee joint region are obtained by means of MRI or CT scan prior to surgery and a custom-made bone resection block for the patient is made based on the image data. During surgery, the bone resection block is mounted to the distal end of the femur and bone resection is performed without checking the position of the femoral head center. This shape matching technique does not require use of an expensive navigation system and can therefore save surgical expense. However, this technique has difficulty in achieving a high accuracy in bone resection, because the bone resection block is blindly mounted on the distal end of the femur without checking the position of the femoral head center. Therefore, it cannot be determined whether or not the bone resection block is in a correct position, based on the position of the femoral head center.